Payback

Each bill's savings are what the bill would have been without solar, minus what you paid (realized), plus the value of credits banked with the utility that month (banked): est_without_solar − bill_amount and (nem_end − nem_start) × rate. So far that's $13,124 realized and $5 banked, through Sep 22, 2026; a month joins the total when its bill is parsed. A bill that drew on banked credit saves less than zero. Credit still banked at the annual true-up is paid out at the utility's as-available rate, a few cents a kWh, so the kWh a payout clears count at that rate, where they were banked and where they left, and the payout itself is in the bill that credited it. Hatched bars are bills whose solar and house figures are estimated or partly metered.

The in-progress estimate fills the gap since the last bill by valuing daily battery-system data at the latest rate: (home − import) × rate realized, export × rate banked. It reconciles when the next bill lands.

The system cost sums every row in the cost ledger (3 entries), stepping up on the chart on the day each was paid.

The pace is the last 12 bills as if they happened today. As billed they saved $5,082; at today's all-in rate, $0.158/kWh, instead of each bill's own, $4,421 (the projection adds rate growth from today, so an older bill's rate would count it twice). Each bill made by a smaller array then gets the solar today's 23.31 kW would have made, 6,842 kWh in all. Only 3,393 kWh of it offsets energy those bills bought (3,393 kWh imported net of exports) and counts at $0.158/kWh; the other 3,449 kWh would be surplus credit. Florida's net-metering rule (25-6.065, F.A.C.) carries surplus credit from month to month and pays out what is left at the annual true-up at the utility's as-available energy rate, taken as $0.030/kWh (the surplus_credit_rate assumption). That comes to $5,060, or $422 a month. The lifetime average ($365 a month over 36 bills) includes an inverter outage and the smaller system, so it isn't used.

Each case compounds its monthly pace a year at a time by (1 + rate growth)(1 − 0.50% degradation): best 5.0% rate growth with output +10%; likely 3.0% rate growth with output as now; worst 1.0% rate growth with output −10%. The dashed line is the likely case (3% a year), the band runs from worst to best, and the marker is where the likely line crosses the cost, labelled with the years of the best and worst crossings. Net after 10, 15 and 25 years is projected savings minus the cost, counted from today: likely, then worst to best. It doesn't count the federal tax credit or year-end credit true-ups. You're 3.0 years in.

The rate is all-in: (bill − customer charge) ÷ billed kWh, so taxes, fees and riders that grow with usage are included, and it is what a kWh from the grid costs and what solar saves. A bill with billed kWh gives its own rate (a filled point). A bill where net-metering credits covered everything (billed kWh 0) has nothing to divide by, so its rate is assumed (a hollow point, 17 bills): the bill's own energy rate × the median all-in ÷ energy ratio when the parser recorded one, otherwise the last rate from a bill; 2023–24 use year defaults. The step line joins only neighbouring bills that both have their own rate. Dotted rules mark bills where the bill parser saw the energy-only rate change. The first year is the 2023 bills with their own rate (12), averaging $0.195/kWh. Pages price new kWh at $0.158/kWh.

Fixed is the parsed customer-charge line; when a bill has no parsed split it's worked out (hatched, 7 bills): bill − rate × billed kWh, or the whole bill on a minimum bill. Variable is the rest, so the two always add up to the bill.

81.5 MWh of solar since install avoided about 28.9 t of CO₂, as much as 477 trees take up in ten years, or 33.2 t of coal not burned. Solar covered 94% of house use over that time. CO₂ at 0.355 kg CO₂ per kWh, EPA eGRID2023 FRCC; trees and coal from the EPA Greenhouse Gas Equivalencies Calculator (16.5 trees per tonne of CO₂, 0.000408 t of coal per kWh).